Some printing processes, for example, that generate three-dimensional (3-D) objects, convert a model of a 3-D object into a format suitable for printing. A printable format may, for example, be a format that represents portions or aspects of the 3-D object by a plurality of units of material or units of a combination of material, for example, dots which can be printed by drops of a fluid. For example, in the generation of 3-D objects, generated by an additive manufacturing process, layers of the 3-D objects are formed by building up corresponding layers of a build material. In one example of additive manufacturing, the object is generated on a layer-by-layer basis by coalescing and solidifying portions of a layer of build material. In one example the build material may be in the form of a powder, fluid or sheet material. The correct coalescence and solidification is achieved by printing onto the layer of the build material an agent. The layer is heated and the build material which is coated with the agent coalesces and solidifies upon cooling.